The present invention relates to interferometric equipment for detecting substances and more particularly the invention relates to interferometric equipment for detecting a substance under utilization of a structured, particularly a periodic or quasi-periodic absorption spectrum and using a source of radiation, a radiation path into which the substance to be investigated is placed, further using an interference filter having a thickness determined by the spacing of relevant interference line and further including a detector.
Interferometric equipment of the kind to which the invention pertains is for example shown German patent no. 26 04 471. The particular device here includes structure for the detection of a substance with a particular characterizable optical path difference and here on uses an assembly that includes the following elements and components: a polarizer, a double refracting plate or platelet having a thickness equal to the characteristic optical path length difference of the substance to be detected and further including a second polarizer operating as an analyzer. This device requires continuous rotation of the analyzer and, therefore, is generally subjected to mechanical wear. Moreover, this particular kind of motorically driven structure limits the application of the equipment and particularly reduces the lifetime as well as increases the extent of maintenance. This is often not desirable particularly in the case of monitoring and inspecting industrial processes.
An interferometric equipment operating with interference filters is also described by J. J. Howarth and H. M. Stanier in "Journal of Scientific Instruments", 1965, volume 42, pages 526-528. Here an infrared process analyzer is used having a second interference filter of a higher order namely a Fabry-Perot filter in a definite manner that is attuned to a periodic absorption spectrum of the substance to be detected. One of the filters is a measuring filter and the other one is a reference filter. For time multiplexing acquisition of reference signals one uses a chopper wheel which again is a mechanically moving device so that this particular interferometric structure is again subject to wear and is limited in its application for reasons mentioned above.